Abstract

Have you ever looked at sunlight through a prism? If so, you know that the prism can separate the sunlight into many different colors of light — a rainbow. Like sunlight, chemical mixtures can also be broken into their component parts. One way of doing this is a simple technique called paper chromatography. What do you think you will see if you use paper chromatography to look at the components of black ink? Is black ink just black? Find out for yourself!

Objective

The objective of this project is to use paper chromatography to analyze ink components in black markers/pens.

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Introduction

What color is black ink? Sounds like a trick question doesn't it! But sometimes things are not just what we
think they are. Often things can be broken down into component parts. For example, milk looks like one thing
but it is actually made up of several components including water, fat, and protein. Which brings us back to
black ink — it looks like one thing, but is it actually made up of more than one component? One way to
find out it to use a common chemistry technique called chromatography.

Chromatography is a technique used to separate the various components in a complex mixture or
solution. There are two parts: a stationary phase and a mobile phase. The stationary phase
does not move. It is the platform on which you put the mixture you want to analyze on. The mobile phase
does exactly what you would expect given the name — it moves. It sweeps the components in the mixture along
the stationary phase separating them by how much they "stick" to each other. It is also referred to as the
solvent.

In this science project you will use paper chromatography to see if black ink can be separated into
components. The ink will be spotted onto strips of filter paper and put in a beaker containing a solution of
alcohol and water. The paper is the stationary phase and the alcohol and water solution is the solvent (mobile phase).
The solvent will move by capillary action. The attraction of the solvent to the paper
(adhesion force) is larger than the attraction of the solvent to itself (cohesion force), hence
the solvent moves up the paper. The ink will also be attracted to the paper, to itself, and to the solvent
differently, and thus a different component will move a different distance depending upon the strength of
attraction to each of these objects. As an analogy, let's pretend you are at a family reunion. You enjoy
giving people hugs and talking with your relatives, but your cousin does not. As you make your way to the
door to leave, you give a hug to every one of your relatives, and your cousin just says "bye." So, your cousin
will make it to the door more quickly than you will. You are more attracted to your relatives, just as some
chemical samples may be more attracted to the paper than the solvent, and thus will not move up the solid phase
as quickly. Your cousin is more attracted to the idea of leaving, which is like the solvent (the mobile phase).

In paper chromatography, you can see the components separate out on the filter paper and identify the components based on how far they travel.
To do this, we calculate the retention factor (Rf value) of each component. The
Rf value is the ratio between how far a component travels and the distance the solvent travels from a common starting point (the origin). For example, if one of the sample components moves 2.5 centimeters (cm) up the paper and the solvent moves 5.0 cm, as shown in Figure 1 below, then the
Rf value is 0.5. You can use Rf values to identify different components as long as the solvent, temperature, pH, and type of paper remain the same. In Figure 1, the light blue shading represents the solvent and the dark blue spot is the colored solution sample.

Figure 1. values are how different components are compared to each other in paper chromatography.

Rf values are calculated by looking at the distance each component travels on the filter paper compared to the distance traveled by the solvent front. This ratio will be different for each component due to its unique properties, primarily based on its adhesive and cohesive factors.

When measuring the distance the component traveled, you should measure from the origin (where the middle of the spot originally was) and then to the center of the spot in its new location. To calculate the
Rf value, we then use Equation 1 below.

Equation 1:

[Please enable JavaScript to view equation]

In our example, this would be:

[Please enable JavaScript to view equation]

Note that an Rf value has no units because the units of distance cancel.

Chromatography is used in many different industries and labs. The police and other investigators use
chromatography to identify clues at a crime scene like blood, ink, or drugs. More accurate chromatography in
combination with expensive equipment is used to make sure a food company's processes are working correctly and
they are creating the right product. This type of chromatography works the same way as regular chromatography,
but a scanner system in conjunction with a computer can be used to identify the different chemicals and their
amounts. Chemists use chromatography in labs to track the progress of a reaction. By looking at the sample spots
on the chromatography plate, they can easily find out when the products start to form and when the reactants have
been used up (i.e., when the reaction is complete). Chemists and biologists also use chromatography to identify
the compounds present in a sample, such as plants.

In this science project you can use a simple paper chromatography setup to see if black ink is just one
component or a mixture of several components. Will the answer be the same for all types of black ink?

Terms and Concepts

Chromatography

Solution

Stationary phase

Mobile phase

Solvent

Capillary action

Adhesion force

Cohesion force

Questions

Why do different compounds travel different distances on the piece of paper?

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Experimental Procedure

To make sure you can compare your results, as many of your materials as possible should remain constant. This means that the temperature, type of water used, size of paper strips, where the ink is placed onto the paper etc. should remain the same throughout the experiment.

Cut the filter paper into strips approximately 2 centimeters (cm) wide by 6.5 cm long. Prepare a total of 15 chromatography strips this way.

Science Buddies Kit: The kit comes with 20 long strips of filter paper; two 6.5 cm strips can be cut from each long strip.

Take one of the chromatography strips and use a ruler and pencil to draw a line across it horizontally 1 cm from the bottom. This is the origin line, see Figure 2 below for details. Repeat this step for all 15 of the chromatography strips.

Figure 2. Each chromatography strip will have an origin line. The pen/marker ink to be tested will be spotted in the middle of the origin line.

Using one of the pens/markers, place a small dot of ink at the center of the origin line of a chromatography strip. This is your spotted sample as shown in Figure 3 below.

Use a pencil to label which pen/marker you spotted on the chromatography strip. Do not use a pen labeling the strips: the ink will run when the solvent passes through the strips.

Repeat this step until you have spotted ink on 5 chromatography strips for each pen/marker.

Figure 3. A marker or pen should be used to put a single spot of black ink in the middle of the origin line on the chromatography strip.

Make a 50% isopropyl alcohol solution to use as your chromatography solvent.

Pour 20 milliliters (mL) of 99% isopropyl alcohol into the 100 mL beaker. Add 20 mL of water to the beaker so that the final volume is 40 mL. Stir thoroughly with the wooden splint.

Pour the 40 mL of approximately 50% isopropyl alcohol solution into the 500 mL beaker. Cover the beaker with plastic wrap, so that the solution does not evaporate. This is your solvent.

Pour about 8 mL of the solvent back into the 100 mL beaker and run two prepared chromatography strips in the beaker.

Clip two of the prepared chromatography strips to a wooden splint. Make sure the two strips do not touch each other and the bottoms align. Rest the splint on top of the beaker so that the strips hang into the jar and do not touch the sides of the jar.

If necessary, add more solvent to the small beaker. The goal is to have the end of each chromatography strip just touching the surface of the solvent solution as shown in Figure 4 below. Add solvent as needed to achieve this goal.

Cover the top of the beaker with plastic wrap.

Set aside the remainder of the unused solvent (covered with a lid or plastic wrap) for additional runs.

Figure 4. The edge of the chromatography strips should just barely touch the solvent. Remember to cover the top with plastic wrap so that the solvent does not evaporate.

Let the solvent rise up the strip (by capillary action) until it is about 0.5 cm from the top, then remove the strip from the solvent. Keep a close eye on your chromatography strip and the solvent front — if you let it run too long the dye may run off the paper and become distorted.

Use a pencil to mark how far the solvent rose.

Allow the chromatography strip to dry, then measure (in centimeters) and calculate the Rf value for each pen/marker dye component. Record your results in your lab notebook.

Tip: The equation for calculating the Rf value is given in the Introduction (located in the Background tab).

Repeat steps 5 - 8 until you have run all of the chromatography strips.

Each time you run the experiment make sure there is enough solvent in the beaker. The chromatography strips should be just touching the surface of the solvent. Add more solvent (50% alcohol solution) as needed.

Using the five repeated strips for each pen/marker, calculate the average Rf for each dye component.

Questions

Did the inks from the different pens/markers separate differently? By looking at the Rf values, can you tell if any of the ink components from the different pens/markers are the same?

If the ink components separated differently for each marker, why did this happen? Hint: Think about the strength of the attractions.

Troubleshooting

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Variations

You could try using other solvents than water (or mixture of solvents) and see if the inks are separated differently (rubbing alcohol, vinegar, nail polish remover, and turpentine would be good to try). Which solvent separates the ink the best? Why? (Remember to account for attractions between chemicals.)

Frequently Asked Questions (FAQ)

If you are having trouble with this project, please read the FAQ below. You may find the answer to your question.

Q: What kind of paper will work for doing this science project?

A: This project works best with high quality filter paper, like that found in the Science Buddies Store or specialty chromatography paper (which is more expensive). Coffee filters and regular printer paper will not work. It is possible to use paper towels to just see the colors separate, but the degree of separation is low and the results will be hard ro quantify. For more details, see the Science Buddies webpage on
Paper Chromatography Resources.

Ask an Expert

The Ask an Expert Forum is intended to be a place where students can go to find answers to science questions that they have been unable to find using other resources. If you have specific questions about your science fair project or science fair, our team of volunteer scientists can help. Our Experts won't do the work for you, but they will make suggestions, offer guidance, and help you troubleshoot.

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